Conventionally, as disclosed in, for example, Japanese Patent No. 3057413, an automatic ship-steering device has been known. The disclosed automatic ship-steering device is configured such that a remote operation box is connected to a control and arithmetic device. The remote operation box has a joystick lever for setting the direction of movement of a hull in accordance with the direction of tilt of the lever and for setting the speed of movement of the hull in accordance with the angle of tilt of the lever, and a swing dial for setting the direction of swinging of the hull in accordance with the direction of turning of the dial and for setting an angular speed of swinging in accordance with the quantity of turning of the dial. During dead slow run, which is achieved by rotating propellers at dead slow speed by means of a speed reducer, a ship operator can turn the hull, cause the hull to undergo pinwheeling, or move the hull translationally by operating only the joystick lever and the swing dial.
The above-mentioned automatic ship-steering device allows a ship operator to turn the hull, to cause the hull to undergo pinwheeling, or to move the hull translationally at dead slow speed by operating only the joystick lever and the swing dial, without need to operate other equipment. Thus, for example, when the automatic ship-steering device is applied to a large ship, since, usually, a seaman who is familiar with behavioral variations of the ship steers the ship, use of the automatic ship-steering device improves operability and allows the seaman to readily move the hull. However, the joystick lever and the swing dial must be operated individually. Therefore, when the automatic ship-steering device is applied to a small ship, there arises a case where a user who is unfamiliar with the behavioral variations of the ship steers the ship. In such a case, the user may fail to readily move the hull by operating the joystick lever and the swing dial. In some cases, a small ship is not equipped with a speed reducer. Such a small ship encounters difficulty in maintaining dead slow run. This may make steering more difficult.
As for a device for maintaining dead slow run, there has been known a slip ratio adjuster for a marine gear as disclosed in Japanese Patent Application Laid-Open (kokai) No. H7-196090. The slip ratio adjuster has a controller for controlling a solenoid which hydraulically controls a clutch of the marine gear, via a PID control circuit and a PWM control circuit. The controller has a variable resistor. A control-range-adjusting trimmer is connected in series with the variable resistor. Thus, even when a ship operator sets the maximum rotational speed of a screw (propeller) to an arbitrary value, a voltage corresponding to the rotational speed of the screw (propeller) can be divided through adjustment of the control-range-adjusting trimmer. By controlling the solenoid with the divided voltage, the full range of the controller can be modified so as to correspond to the set maximum rotational speed. Therefore, fine adjustment of rotation of the screw (propeller) can be carried out with ease.
However, according to the conventional adjuster for adjusting the slip ratio of the marine gear as mentioned above, the slip ratio is adjusted merely proportionally in association with modification of the full range of the controller, thereby adjusting the rotational speed of the screw (propeller). That is, the slip ratio is not adjusted in consideration of ship behaviors. Therefore, for example, even when a ship operator is to run a ship at dead slow speed by operating the controller, the screw (propeller) may fail to actually rotate due to influence of disturbances, such as a resistance force against rotation of the screw (propeller), waves, and wind. Thus, the speed of movement of the ship varies nonlinearly in relation to a control input to the controller, and dead slow run intended by the ship operator may fail to be maintained. Therefore, the ship operator must control the controller at all times, so that ship-steering operations become complicated.